A multi fuel engine refers generically to any type of engine, boiler, heater or other fuel-burning device which is designed to burn multiple types of fuels in its operation. Multi fuel engines have application in diverse areas to meet particular operational needs in the operating environment. For example, a common use of multi fuel engines is in military vehicles so that vehicles in various deployment locations may run a wide range of alternative fuels such as gasoline, diesel or aviation fuel. In combat settings, for example, enemy action or unit isolation may limit the available fuel supply and personnel may need to resort the type of fuel available for usage from enemy and civilian sources. Multi fuel engines are also desirable where cheaper fuel sources, such as natural gas, are available, but an alternative or secondary fuel, such as diesel fuel, is needed for performance reasons (e.g., faster reaction to short term load demand), as a backup in the event of an interruption in the supply of the primary fuel source, or for other operational or engine performance conditions.
A multi fuel engine typically operates with a specified mixture of the available fuels. Where a liquid-only fuel mixture is specified, a liquid fuel, such as diesel fuel, gasoline or other liquid hydrocarbon fuel, is injected directly into an engine cylinder or a pre-combustion chamber as the sole source of energy during combustion. When a liquid and gaseous fuel mixture is specified, a gaseous fuel, such as natural gas, methane, hexane, pentane or any other appropriate gaseous hydrocarbon fuel, may be mixed with air in an intake port of a cylinder and a small amount or pilot amount of liquid fuel, such as diesel fuel, is injected into the cylinder or the pre-combustion chamber in an amount according to a specified substitution ratio in order to ignite the mixture of air and gaseous fuel.
Machines having multi fuel engines are configured with fuel substitution strategies that control the mixture of liquid and gaseous fuels to provide the necessary power for the engine during operation of the machine. For example, a fuel substitution strategy may specify a mixture of 80% gaseous fuel and 20% liquid fuel during normal operations. At times, the fuel substitution strategy may adjust the substitution ratio to increase the percentage of liquid fuel or gaseous fuel as necessary to meet short term deviations in the power requirements for the engine, known as transient events, such as when the load on the engine or the speed of the machine increases or decreases. In current fuel substitution strategies, the machines react to the occurrence of the transient events, and therefore lag in responding to the transient events with resulting degradation of performance. For example, when an automatic transmission up shifts to a higher gear according to shift control logic programmed into the machine, the engine experiences a transient event in the form of an increase in torque after the up shift occurs. A substitution ratio that is too rich in gaseous fuel that requires an increase in the intake air to maintain an air fuel ratio (AFR) may experience a power loss during and after the transition to the higher gear until the intake air catches up to the new fuel demand or the substitution ratio adjusts to a mixture that is richer in the liquid fuel. In view of this, a need exists for improved control over fuel substitution ratios when encountering various operating conditions that may cause transient events that can be anticipated.